1. Field of the Invention
The present invention relates to image processing devices comprising a sheet feeding device configured to feed sheets, such as documents and recording mediums, using a plurality of rollers.
2. Description of Related Art
Known scanners, facsimile machines, and copiers, and multifunction devices configured to perform the functions of a scanner, a facsimile machine, and a copier, such as the devices described in U.S. Pat. No. 7,080,836 B2 and Japanese Laid-Open Patent Publication Nos. 9-238236 and 5-155459, include a sheet feeding device that automatically feeds documents stored in a document tray, one by one, to a reading position, or automatically feeds recording mediums held in a sheet cassette to a recording position.
The sheet feeding device includes a plurality of rotating shafts which are configured to rotate and have a predetermined distance therebetween in a feeding direction of a sheet. At least one drive roller is mounted to each rotating shaft. When a motor inputs a rotating force to the rotating shaft, the drive roller rotates with the rotating shaft. A driven roller is pressed against a roller surface of the drive roller. Therefore, the driven roller rotates with the drive roller. When the leading end of a sheet reaches a nip position between the drive roller and the driven roller, the sheet is fed by the drive roller in the feeding direction.
Referring to FIG. 15, a known roller supporting mechanism is depicted. In the known roller supporting mechanism, first drive rollers 201 are disposed upstream of second drive rollers 202 in the feeding direction, as indicated by an outline arrow. A rib 207 is disposed at each end of a feeding path 206 in its width direction. First drive rollers 201 are mounted to a rotating shaft 211. The ends of the rotating shaft 211 are rotatably supported by ribs 207. Second drive rollers 202 are mounted to rotating shaft 212. The ends of the rotating shaft 212 are rotatably supported by ribs 207.
When the leading edge of a sheet 208 reaches first drive rollers 201, first drive rollers 201 feed sheet 208 in the feeding direction. After the leading edge of sheet 208 passes first drive roller 201 and reaches second drive rollers 202, second drive rollers 202 feed sheet 208. After the leading edge of sheet 208 reaches second drive rollers 202 and before the trailing end of sheet 208 passes over first drive rollers 201, both first drive rollers 201 and second drive rollers 202 feed sheet 208. When the circumferential velocity of first drive rollers 201 is greater than the circumferential velocity of second drive rollers 202, sheet 208 flexes between first drive rollers 201 and second drive rollers 202, and may be damaged. Therefore, the circumferential velocity of second drive rollers 202 disposed on the downstream side in the feeding direction is adjusted to be slightly greater than the circumferential velocity of first drive rollers 201. Accordingly, a first feeding force with which first drive rollers 201 feed sheet 208 is greater than a second feeding force with which second drive rollers 202 feed sheet 208, such that sheet 208 is held under tension between first drive rollers 201 and second drive rollers 202.
The known sheet feeding device only supports rotating shafts 211 and 212 at their ends. Consequently, the difference between the circumferential velocity of first drive rollers 201 and the circumferential velocity of second drive rollers 202 causes a tensile force. The tensile force pulls sheet 208 while generating a frictional force which draws rotating shafts 211 and 212 closer to each other. The frictional force causes rotating shafts 211 and 212 to flex toward each other, as indicated by dotted lines in FIG. 15. After the trailing end of sheet 208 passes over first drive rollers 201, flexed rotating shafts 211 and 212 return to their original positions. At this time, sheet 208 moves in the feeding direction by an amount corresponding to an amount that flexed rotating shaft 212 of second drive rollers 202 moves from its original position when shaft 212 flexes. As a result, a feeding failure occurs and places the document out of position with respect to the feeding direction. When a feeding failure occurs during the process of reading an image recorded on sheet 208 or during the process of recording an image on sheet 208, read image data or recorded sheet 208 will have streaks. Thus, the accuracy of reading an image or recording an image is reduced.
When sheet 208 is stopped intermittently while being fed by first drive rollers 201 and second drive rollers 202, a feeding failure occurs due to various factors, such as non-uniform friction forces applied to first drive rollers 201 and second drive rollers 202 when the sheet feeding is temporarily stopped and resumed, non-uniform amounts of flexing of rotating shafts 211 and 212 at the timing of stopping and resuming of the sheet feeding, or non-uniform flexing amounts of an elastic member, such as rubber, at the time of stopping and resuming of the sheet feeding if an elastic member is disposed or applied on a surface of drive rollers 201 and 202. Such feeding failure causes streaks in the read image data or in an image recorded on a sheet 208. Thus, the accuracy of reading an image or recording an image is reduced.
The above problems occur on small-diameter rotating shafts used to reduce the weight of the sheet feeding devices. The problems also occur on image reading devices having auto document feeders (ADF) that feed large-sized documents, such as A3 size, or on inkjet-type image recording apparatuses configured to record an image on large-size recording sheets, such as size A3, because the amount of flexing of rotating shafts 211 and 212 increases in these devices.